Heating system and boiler therefor

ABSTRACT

A method of heating a building in which a boiler provides hot water for circulating through radiators in the building and a condensing heat exchanger recovers heat from the boiler flue gases. Air entering the building is heated by an additional heat exchanger through which water flows which has been heated by the condensing heat exchanger. Cold air supplied to the combustion chamber is also employed to cool the flue gases. The condensing heat exchanger comprises a first exchanger in which the hot flue gases are cooled to a temperature above the dew point and a second in which the previously cooled flue gases are cooled to below the dew point. The second exchanger includes a primary water circuit which cools water in a secondary circuit, in which water is in contact with the flue gases and the secondary circuit water passes through a heat exchanger in an air inlet to the building. Some of the water returning from the radiators is cooled before supply to the primary circuit. A building and heating system comprises a boiler which provides a source of hot water, a pump for circulating the hot water through radiators in the building, a condensing heat exchanger which recovers heat from the boiler flue gases, and in which water which has been heated in the condensing heat exchanger is employed to heat air entering the building.

FIELD OF INVENTION

The invention concerns heating systems of the type comprising a water heating apparatus (commonly called a boiler) to provide a source of hot water for circulating through radiators in a building to be heated and a secondary heat exchange apparatus for recovering heat from the products of combustion from such apparatus. The combination of water heating apparatus and secondary heat exchange apparatus is called a condensing boiler if the heat exchange apparatus removes latent heat of evaporation from water vapour in the products of combustion, and the present invention is applicable to condensing boilers. The invention also concerns methods of operating condensing boilers and heating systems for buildings containing such boilers and to secondary heat exchange apparatus for use in such condensing boilers.

The fuel to be burnt to heat water in the boiler may be gaseous such as natural gas or even hydrogen, or in the case of oil/paraffin/kerosines may be a liquid. However the invention is not limited to the use of any particular fuel and is applicable to boilers which burn any fuel.

BACKGROUND TO THE INVENTION

A condensing boiler based heating systems will cause the steam of combustion of a fuel to condense to liquid water and will also collect the latent heat of vaporisation of the steam and recycle this heat into the boiler system and thus increase the thermal efficiency of the boiler system. See for example UK Patent Specification 2382403, which describes such a condensing boiler and a heating system using such a boiler.

The following points have been noted in relation to water boilers:—

1. The flue gas temperature from a boiler without a condensing secondary exchanger is well above 100° C., typically in the range 150°-250° C., and it is quite certain that such a boiler recovers no latent heat from steam because the flue gas temperature is too high for any condensation to take place. If then, such a boiler operates with a gross efficiency of nearly 90%, then it is possible to conclude that all the steam latent heat is still trapped in the flue gases effluent from the boiler, and will escape up the flue.

2. The same boiler, fitted with a stainless steel (or other suitable metal) secondary heat exchanger as previously proposed, will demonstrably operate with a gross efficiency of about 93%, an efficiency gain of just 3%. This secondary exchanger, although fitted with a water condensate overflow tube, has been found to provide only a litre or so of water over an operating time of several hours. Therefore the secondary exchanger (condenser) described, although condensing some water and therefore correctly termed a condensing heat exchanger, is only condensing a small proportion of the steam, and a proper description would be a partially condensing heat exchanger because in no way is the volume of water collected representative of the volume of steam generated by the burning fuel, but is only a fraction thereof.

3. As confirmation of (2) above, large quantities of steam can be seen issuing to atmosphere from the secondary exchanger previously proposed, and this shows that at any rate some steam escapes the secondary exchanger and latent heat escapes with the steam.

4. In a typical example of a kerosene based fuel, the percentage of hydrogen in the fuel by weight is about 13.74%. Thus it can be shown that burning 1 US gallon per hour of the fuel will produce 3.6 kilograms of water as steam per hour. Moreover, the specific latent heat of steam at 100° C. is 2.26 megajoules per kilogram, or almost 6% of the total calorific value of the fuel, and so far only a fraction of this has been recovered by secondary heat exchangers as proposed. For total latent heat recovery to take place, it follows that about 3.6 kilograms of water could (and should) be recovered. In practice perhaps as little as 10% is actually recovered. Therefore although a 3% efficiency gain has been achieved, the principal amount of energy recovered by the previously proposed secondary heat exchanger is due to reducing the temperature of boiler exhaust gases. The bulk of the steam and its energy escapes to atmosphere, as evidenced by the small volume of condensed water collected

5. The dew point of the water vapour component in the flue gas of a typical domestic boiler is about 50°-55° C. and can be lower. It is quite impossible to fully condense this water and to capture the energy of condensation unless the medium used to cool the gases is below, and ideally considerably below, the dew point. The temperature of the return water from the radiators previously proposed to cool the flue gases is at about 60° C., and therefore is unavailable to cool the flue gases below the dew point.

6. It is possible to reduce the water flow through the radiators so that the return temperature is sufficiently low (e.g. 25°-30° C.). This would allow the returning water to be employed in the heat exchanger to effect full condensation of the water vapour in the still hot gases, but in practice the very low flow rate through the radiators is not practicable.

In the case of combustion of north sea gas, (natural gas) the latent heat in the steam content of the exhaust gases can account for approximately 10% of total combustion energy.

In the case of liquefied gases such as butane and propane, the latent heat in the steam content of the exhaust gases can account for about 8% of total combustion energy.

In the case of lighter fuel oils, such as Kerosene and Gas Oil, the latent heat in the steam content of the exhaust gases can account for about 6% of total combustion energy.

These figures are based on typical content of hydrogen of the fuels. The latent heat can be taken to be proportional to the hydrogen content of the original fuel since the latent heat in the exhaust gases will be equal to the heat required to vaporise the water which results when the hydrogen component is burnt in air.

OBJECT OF THE INVENTION

It is an object of the present invention to provide further improvements to the condensing boiler and associated heating system described in UK Patent Specification 2382403 so as to provide a more efficient method and apparatus for recovering the latent heat in the water vapour present in the flue gases, and an improved condensing heat exchanger for such a boiler, and an improved method of heating a building using such a boiler.

SUMMARY OF THE INVENTION

According to one aspect of the present invention in a method of recovering latent heat from water vapour in the flue gases from a hydrocarbon fuel-burning boiler adapted to heat water to be circulated through radiators to heat a building, in which hot flue gases from the boiler are cooled first by a heat exchanger through which water returning from the radiators is caused to flow and then by a second heat exchanger through which water at a temperature typically in the range 20° to 40° C. so as to be below the dew point of the water vapour in the flue gases is caused to flow in direct contact with the flue gases, so that not only is heat extracted from the latter but soluble products of combustion (such as SO₂) are dissolved in the water and are thereby separated from the flue gases before they exit to atmosphere, and wherein before the water returning from the radiators is supplied to the first heat exchanger a small proportion of the returning water is diverted through an additional heat exchange device located in a relatively cool region in the building being heated, and thereafter the diverted water is further cooled as it passes through an air to water heat exchanger through which air which is to support combustion in the burner of the boiler passes en route to the combustion chamber, such that the exit water temperature therefrom will be in the range 20°-40° C. so as to be below the said dew point and the exiting water is supplied to cool water in the second heat exchanger.

The second heat exchanger may include a subsidiary water to water heat exchanger through which the said exiting water flows to cool water which flows around a closed circuit defined by the second heat exchanger, and after passing through the subsidiary heat exchanger the water is returned to the boiler.

Preferably the water leaving the subsidiary heat exchanger is mixed with water returning to the boiler from the first heat exchanger before the latter is returned to the boiler.

The invention also lies in a boiler for supplying hot water to radiators in a building having first and second heat exchange means for cooling the exhaust gases through the first of which water returning from the radiators is caused to flow before returning to the boiler, which includes a water path by which a fraction of the water returning from the radiators to the first heat exchanger is first caused to flow direct to an additional heat exchange device instead of to the first heat exchanger, and after passing therethrough to pass through an air to water heat exchanger in an air inlet to the boiler supplying air to support combustion therein, thereafter to pass through a water to water heat exchanger adapted to cool water circulating in the said second heat exchanger, and lastly to return the water to a water inlet to the boiler, where it is mixed with water passing thereto from the first heat exchanger.

Preferably the said water path includes valve means for adjusting the proportion of the water returning from the radiators which travels therealong and the proportion which proceeds to the said first heat exchanger.

The said additional heat exchange device may be a conventional panel radiator or a fan assisted radiator.

The said first and second heat exchangers may be located in a common housing with the first above the second and spaced vertically therefrom, and the hot flue gases from the boiler flue are directed into an opening at the top of the housing to pass over and around an enclosure forming the first heat exchanger through which water returning from the radiators is caused to pass, thereby to cool the gases, before they proceed to be further cooled by the said second heat exchanger, wherein baffle means is provided below the said opening to direct some of the incoming gases sideways.

It has been found advantageous if the first heat exchanger is comprised of two enclosures arranged side by side with a gap between them and between their outer faces and the internal walls of the housing, to allow for the passage of hot gases therearound as they pass in a downward direction through the housing.

Preferably the interior of the or each enclosure is divided into at least two regions by means of at least one baffle plate.

Conveniently the or each baffle plate extends partly across the interior of the or each enclosure so as to leave a gap at one end between it at the internal surface of a wall of the enclosure, but sealingly engages with the internal surface of each of the other three walls of the enclosure, the gap allowing water to pass from one region to the other.

Alternatively the or each baffle plate extends completely across the interior of the or each enclosure and an opening is provided at one end in the baffle plate to allow water to pass from one region to the next. Typically at least two baffle plates are provided, spaced apart within the or each enclosure, and the gaps or openings are located alternately at different sides of the enclosure, so as to create a tortuous path for water therethrough.

In a preferred arrangement there are four baffle plates defining five regions through which the water flows.

Typically the water flows in parallel through the two enclosures.

The second heat exchanger typically comprises an enclosure through which cool water flows after leaving the air to water heat exchanger in the burner air inlet, which is spaced from the walls of the housing near the base thereof, the latter forming a reservoir for water which has cascaded down through a space between the first and second heat exchangers in the housing to mix with and cool the flue gases as they descend through the housing.

The enclosure of the second heat exchanger may include at least one baffle plate to divide the interior into two regions, with a gap or opening at or near the end of the or each baffle plate to allow water to flow from one region to the next. Typically at least two such baffle plates are arranged so that the gaps or openings are alternately at different sides of the enclosure, to create a tortuous water path therethrough.

Conveniently the or each baffle plate includes tongues which extend laterally of edges thereof and the enclosure walls include slots therein through which the tongues pass when the plates are fitted therein and in which they are welded or bonded by an adhesive.

Preferably the baffle plates are of thermally conductive material and make good thermally conductive connection to the walls of the enclosure(s).

Advantageously the baffle plates additionally serve to brace the enclosures.

The invention also lies in a heating system comprising a boiler embodying the invention in combination with radiators connected by pipe means to each other and to the boiler for heating a building in which the radiators and the boiler are located.

The invention also lies in a heating system comprising a boiler embodying the invention and radiators connected to each other and to the boiler for heating two or more separate buildings in each of which some of the radiators are located and in one of which the boiler is located, or in which the boiler is located in a separate building not containing radiators.

A heating system operating according to the invention thus allows the transfer of heat energy from the flue gases to the circulating water, to the air supply to the boiler burner, and via the additional heat exchange device to the building by full condensation of the boiler flue gas water content, collecting as it does the heat energy in the gases and the latent energy from the water content of the flue gases, therefore reducing fuel consumption and/or increasing available energy for heating the building.

The invention also lies in a boiler operating as aforesaid and in such a boiler in combination with a pumped radiator system through which hot water from the boiler circulates to heat the building so as to provide a system which not only recovers the latent energy in the water vapour content of the exhaust gases but recycles this recovered energy back to the boiler and to the building thus reducing the boiler fuel consumption.

The additional heat exchange device may for example be located in a stair-well, entrance lobby or utility room in which a high temperature of the order of 68-72° F. would not be called for, but instead a temperature of the order of 55-60° F. for example.

According to a preferred feature of the invention a secondary heat exchanger and connections to an additional heat exchange device as aforesaid, may be fitted to an existing hydro-carbon fuel burning boiler installation, in particular so as to fit inside a casing for such a boiler.

The invention also lies in a building having a heating system as aforesaid installed therein.

The invention also lies in a local area heating system powered by a condensing boiler as aforesaid in which the radiator system extends to two or more buildings and the additional heat exchange device is located in one of the buildings and the boiler is also located in one of the buildings or in a separate building.

The advantages of the invention may be seen by considering a domestic dwelling in the UK, in winter, when full boiler heat output will be required.

In winter the temperature of the air drawn in to enable combustion to occur in the boiler is typically in the range 0°-5° C. Most domestic boiler flue systems are of the so-called balanced flue type, so that cold air is drawn straight into the boiler burner system. It has been calculated that in the case of an 80,000 BTHu boiler, the mass of this cold air drawn in per hour is about 34 kg. The hourly aspiration of this mass of air at 0°-5° C. affects the boiler heat output since it represents a cooling effect in the combustion chamber and it has been calculated that in such a boiler, the heat output is reduced by approximately 0.5-0.75%.

Another significant heat loss is directly due to the fact that a typical household consumes about 250 litres daily of water. In winter, this water is supplied to the house at a temperature of about 6°-8° C., and this cold water has to be warmed or even boiled for many household uses. The energy required to do this in winter will therefore be higher than in summer when the cold water (especially if stored in the house before use) can be in the range 15°-20° C. Frequently the cold water is stored in a tank in a loft or attic, and unless carefully insulated such tanks can freeze in winter. The invention envisages a system which can help to avoid this problem by providing a water to water heat exchanger in the cold water storage tank and circulating thereto some of the return flow water from the radiator system within the building either directly from the radiators or after passing through the said additional heat exchange device after passing through the air-cooled heat exchanger in the air inlet to the burner of the boiler, and before the cooled water is supplied to the second heat exchanger so as to further cool water to be used to cool the flue gases while simultaneously warming the water stored in the tank, thereby reducing energy needed to heat that water and also reducing the possibility of the reservoir of water freezing in winter. A thermostatically controlled valve can restrict the return flow if the temperature of the water in the storage reservoir exceeds a preset temperature.

It has been calculated that the overall efficiency of an installation comprising a boiler operating at say 15 kw in combination with a closed loop water/hot gas (CLW/HG) system will be enhanced by typically from 9% to 15%. Put another way, the boiler fuel consumption will fall by a similar amount (i.e. typically from 9% to 15%), thus reducing CO₂ emissions by the same amount.

According to another aspect of the present invention, a method of heating a building in which a water heating boiler provides a source of hot water for circulating through radiators in the building and a secondary condensing heat exchange means operates to recover heat from the products of combustion from the boiler, and in which there is an air inlet to the building comprising the step of warming air entering the building by a heat exchanger in the air inlet through which water is caused to flow and reheating the water in the secondary heat exchange means using heat from the hot products of combustion, thereby to transfer heat therefrom to air entering the building.

Preferably the secondary heat exchange means operates to lower the temperature of the products of combustion below the dew point of water vapour therein thereby to gain the latent heat of evaporation of the water content of the products of combustion.

The air inlet to the building may also comprise an air intake for supplying air to the combustion chamber of the boiler, so that after passing through or around the heat exchanger some of the warmed air goes to the combustion chamber in the boiler to support combustion and the remainder of the warmed air is diverted into the building, to directly warm the latter.

Alternatively the air inlet to the boiler combustion chamber may be separate from the air inlet to the building and a heat exchanger may be associated with each of the air inlets and in use heated water is caused to flow through both heat exchangers. Both may be heated by the same re-heated water.

Fan means may be employed to control the flow of air into the air inlet to the building, and where the combustion air inlet is separate therefrom, a separate fan may be employed to control the flow of air for combustion in the boiler.

Where fan means is provided to control the inflow of air into the building and/or for combustion in the boiler, the fan speed may be adjustable to increase and decrease the airflow.

In a preferred arrangement the secondary condensing heat exchange means comprises two separate heat exchangers a first in which the flue gases are cooled by water flowing therethrough to a temperature which is above the dew point temperature and a second through which the previously cooled flue gases pass and in which they are cooled to below the dew point temperature before escaping to atmosphere, and in which the second heat exchanger includes primary and secondary water circuits, water flowing around the secondary circuit being cooled by water flowing through the primary circuit, the secondary circuit water being in contact with the flue gases to cool them below the dew point, and water from the secondary circuit is caused to pass through the heat exchanger in the air inlet, to cool the secondary circuit water and warm the incoming air, before being returned to rejoin the water flowing around the secondary circuit.

In a preferred arrangement water which is to flow through the primary circuit is obtained by diverting some of the water returning from the radiators in the building to the boiler, and this diverted water is cooled by flowing through at least one further heat exchanger before being supplied to an inlet to the primary circuit.

The rate of flow of water diverted as aforesaid in a typical domestic heating system incorporating an 80,000 BThU boiler typically will be in the range 0.5 to 1.0 litres per minute, but the invention is not limited to such a flow rate.

The said at least one further heat exchanger may comprise a radiator located within a region of the building which is maintained at a temperature which is lower than that of other regions of the building and if that is the only device for cooling the diverted water, the region of the building will need to be below the dew point temperature.

Where the building includes a reservoir of cold water for supplying water to a heated water cylinder which is to supply hot water to taps in the building, a further heat exchanger may be provided comprising for example a coil of tubing associated with the cold water reservoir, through which the diverted water may be caused to flow before it is supplied to the inlet to the said primary circuit. Again if this is the only device for cooling the diverted water, the cold water temperature needs to be below the dew point temperature.

The further heat exchanger linked to the cold water reservoir is preferably one in which the two water paths are essentially parallel and the flow of water to be warmed along one path is in the opposite direction to the flow of water which is to do the warming along the other path.

Such a heat exchanger is called a contra flow heat exchanger.

In a preferred arrangement the further heat exchanger comprises two lengths of copper tube one inside the other and defining an annular water path between the two tubes which is closed at each end and includes a water inlet to inner tube at one end and a water outlet at the other end and likewise a water inlet to the annular water path near the outlet from the inner tube and an outlet from the annular path near the inlet to the inner tube, with pipe connections for supplying the diverted water to the inlet and retrieving diverted water from the outlet of the annular path, and a pump for circulating water from the reservoir through the inner pipe to re-enter the reservoir as it exits from the inner tube outlet, thereby to create a contra flow heat exchanger in which the two water paths which are essentially parallel so that in use the flow of water to be warmed along one path is opposite to the direction of the flow of water which is to do the warming and which flows along the other path.

Where both further heat exchangers are to be used to cool the diverted water, preferably the diverted water is caused to flow first through the radiator and thereafter through the heat exchanger associated with the cold water reservoir.

The diverted water returning from the radiators in the building may instead or in addition be forced to pass through a heat exchanger in an air inlet to the combustion chamber, to be cooled thereby to a temperature in the range 0°-10°, and typically to a temperature of the order of 8° C. Again it is preferable for the incoming air to be below the dew point temperature.

Preferably a pump is provided to pump secondary circuit water through the second heat exchanger and through the air inlet heat exchanger.

Preferably only some of the secondary circuit water is circulated through the air inlet heat exchanger before it returns to the secondary circuit, and the remainder of the secondary circuit water is simply circulated around the secondary circuit by the pump.

In the preferred arrangement water in the secondary circuit is pumped to an elevated position and thereafter caused to flow in a downward sense as a cascade, preferably over and around a plurality of baffles, and hot flue gases are caused to pass either in an upward sense through, or downwardly in the same direction as, the cascade in direct contact with the water so that any water and water-soluble products of combustion condensed from the gases can mix with the cascading water and be drained off.

Due to the addition of the condensed water to the water circulating in the secondary circuit the volume of water in the secondary circuit will increase, and overflow means is preferably provided to draw off the excess volume.

A pump may be provided to pump water in the secondary water circuit from a region in which it is cooled by water flowing through the primary circuit, to a spray means, to form the cascade, which pump is separate from the pump which pumps secondary circuit water through the air inlet heat exchanger.

Alternatively one pump may be employed to pump secondary circuit water.

Since water soluble products of combustion leave the gases and mix with the water cascade, these products of combustion are removed from the flue gases.

In the preferred arrangement, the water and gases are separated by gravity.

Where the incoming air is below 20° C. and the air inlet heat exchanger is supplied with water from the secondary circuit of the second heat exchanger, which is at a temperature in the range 20° to 30° C., the incoming air will be warmed, and the secondary circuit water will thereby be cooled before it is returned to the second heat exchanger.

An overall control system may be provided, together with temperature sensors and/or flow sensors and electrically controlled valves for adjusting the volume and/or rate of the flow any diverted water associated with the secondary or the primary circuits. In the case of some installations (such a control may include or comprise a digital computer) and this is particularly the case for larger installations.

Likewise where diverted water is to flow through two or more heat exchange means the control system may be programmed to determine through which of the heat exchangers the water is to flow.

A pump is normally employed to circulate the heated water from the boiler to the radiators and the flow induced by such a pump may be sufficient to cause the diverted returning water to flow through the said further heat exchanger or heat exchangers and primary circuit. Where this is not the case and/or greater control over the flow rate of the diverted returning water is required, a further pump may be provided for pumping the diverted water through the said further heat exchanger or heat exchangers and through the primary circuit.

If incorporated, a control system may control the operation and speed of the or each pump (to control water flow) and/or the speed of operation of any fan in any air inlet.

Where the air inlet to the boiler combustion chamber is separate from the air inlet to the building, a heat exchanger may be located in an air duct leading to the combustion chamber and first valve means may be provided downstream of that heat exchanger for controlling the flow of air through the air duct to the combustion chamber, and a bypass may be provided between an opening in a wall of the air duct upstream of the heat exchanger to an opening in the air duct wall downstream thereof, through which some or all of the incoming air can pass directly to the combustion chamber.

Second valve means may be provided to control the flow of air through the bypass.

The first and second valve means may be preset on installation or may be controlled by a subsidiary control system, which may be part of the overall control system or may be separate therefrom. The subsidiary control system may also be responsive to signals from sensors for sensing air temperature and/or water temperature and/or air flow and/or water flow rates and may also control fan speeds and pump speeds.

Pipes interconnecting heat exchangers for cooling the diverted water and for conveying the cooled diverted water to the secondary heat exchange means are preferably lagged or otherwise insulated, especially where the pipe run(s) is/are significant (as may be the case if a reservoir of cold water is in a roof space of a house and the boiler and secondary heat exchange means are on the ground floor or in a basement of the building).

In one embodiment of the preferred arrangement, the secondary heat exchange means comprises a closed housing divided internally into two compartments, an upper compartment containing a first coiled tube heat exchanger, a flue gas inlet at its upper end, and a flue gas outlet at its lower end, and a lower compartment having upper and lower regions, a second coiled tube heat exchanger forming the said primary circuit in the lower region with water inlet and outlet connections at opposite ends of the coil, and a plurality of baffles defining a tortuous path between the top and bottom of the upper region of the lower compartment, duct means communicating between the flue gas outlet of the upper compartment and a flue gas inlet to the upper region of the lower compartment for conveying flue gases between the two compartments, a flue gas outlet conveying flue gases from the upper region of the lower compartment to atmosphere, pipe means for conveying water from a water outlet at the bottom of the lower region of the lower compartment to an inlet to a water spray means at or near the top of the upper region of the lower compartment, so that in use, water cascading downwardly over the baffles in contact with upwardly moving flue gases following the tortuous upward path in the said upper region of the lower compartment comprising the said secondary circuit, and further pipe means for conveying water from an outlet of the coiled tube in the lower region of the lower compartment to an inlet of the first coiled tube in the upper compartment so that in use the water flowing through the coiled tube in the upper compartment is warmed before it leaves the secondary heat exchange means to return to the boiler.

Alternatively the hot gases can be supplied to the top of the said upper region so as to flow downwardly with the cascade and exit at the bottom.

The invention also lies in a central heating boiler in combination with secondary condensing heat exchange means as proposed herein, when forming part of a heating system for a building, as aforesaid.

The invention therefore also lies in a building when heated by a heating system as aforesaid, and which can be thought of as a large heat sink to which heat is supplied from the combustion process by means of at least one of a plurality of different heat exchange processes involving the steps of, heating air entering the building, heating water which is supplied to radiator heat exchangers in the building, warming cold water stored in the building, and gaining heat from hot gaseous products of combustion by cooling them to a temperature below the dew point of water vapour therein before being exhausted to atmosphere, the heat thereby given up by the hot products of combustion and the latent heat of vaporisation of their water content also being employed to heat water and/or air employed in heating the building.

The invention provides a heating system for a building in which heat is exchanged between low temperature diverted water, and water circulating around a generally closed circuit, to cool the latter to well below the dew point temperature, to achieve condensation of the hot exhaust gas water vapour content, and subsequently warming the diverted water flow before it is to return to the main heat exchanger in the boiler so that the return water flow to the boiler from the radiators is not cooled by the mixing therewith of water the secondary condensing heat exchange means associated with the boiler.

The coiled tube heat exchanger in the upper compartment may be stainless steel for an oil fired boiler, or aluminium for a gas fired boiler.

Typically the diverted water flow entering the coiled tube heat exchanger in the upper compartment is at a temperature of the order of 30° C. and is heated by the hot gases to a temperature of the order of 60 to 70° C. before it is returned to the boiler.

The boiler is typically fitted within a casing, and the secondary heat exchange means may also be fitted within a casing which may be separate from but adjacent, to or located within the boiler casing.

Alternatively the secondary heat exchange means and boiler may be mounted within a single casing.

The invention will now be described by way of example with reference to the accompanying drawings in which:

FIGS. 1 and 2 are end and side elevation diagrammatic views of an improved secondary heat exchanger for use with a water heating boiler, which allows for direct water/flue gas contact;

FIGS. 3 and 4 illustrate alternative arrangements for direct flue gas/water contact.

FIG. 5 is a side elevation diagrammatic view of another heat exchanger,

FIG. 6 is a diagram of a system using a condensing boiler and heat exchange device,

FIGS. 7 and 8 illustrate a modified form of construction for the heat exchanger of FIG. 6,

FIG. 9 illustrates the form and construction of baffle plate used in FIGS. 7 and 8,

FIGS. 10A and 10B are a diagrammatic flow diagram of a boiler and secondary heat exchanger in a radiator heating system constructed as a preferred embodiment of the invention,

FIG. 11 is a diagrammatic cross section through a twin casing secondary heat exchanger constructed as a preferred embodiment of the invention,

FIG. 12 is a diagrammatic view of a different cold water reservoir from that shown in FIG. 1, which incorporates a contraflow heat exchanger,

FIG. 13 is a diagrammatic view of an alternative arrangement for the cold water reservoir and contraflow heat exchanger.

Calculations of Specific Latent Heat of Steam

According to published data, the specific latent heat of steam at 100° C.=2.26×10 kilojoules per kilogram or, more suitable for our calculations=2.26 megajoules per kilogram, (Mj/Kg)

Now in the case of a boiler consuming fuel at the rate of 1 US gallon/hour, this equates to 3.64 litres of fuel/hour.

The weight of fuel consumed per hour is obtained by multiplying the volume by the density of the fuel. The density of one typical fuel is 0.7945. Therefore the weight of such a fuel burnt (per hour)=2.892 kilograms per hour.

If the gross fuel calorific value=46.08 Mj/Kg, then the total calorific value per hour is 2.892×46.08=133.2 Mj per hour.

If, as is typical, the percentage of hydrogen in the fuel=13.75%, then the weight of hydrogen in 2.892 Kg of fuel=2.892×0.1374=0.4 Kg, i.e. 0.4 Kg of hydrogen is oxidised each hour.

Hydrogen reacts with 8 times its weight of oxygen to produce water. Thus the weight of water produced per hour is 0.4×9=3.6 Kg of water per hour, albeit produced in the form of steam.

If the latent heat of steam=2.26 Mj/Kg, the latent heat of 3.6 Kg of steam is 3.6×2.26=8.136 Mj. This is the heat per hour contained in the steam in the flue gases of such a boiler burning this “typical” fuel.

If the total energy of the burnt fuel is 133.2 Mj per hour, then the percentage of the latent heat in the steam to the total energy is 8.136/133.2=6% of the total energy from the fuel.

This figure should be seen in relation to the commonly held belief that the latent heat of steam in the boiler flue gases is approximately 3% of the total calorific value of the fuel burnt.

Also the total weight of steam per hour is 3.6 Kg, which is equivalent approximately to 3.6 litres of water per hour, which by coincidence is similar to the volume of the fossil fuel consumed. This is about 60 cc/mimute.

Modifications to the Secondary Heat Exchanger Proposed by UK Patent Application No. 0107963.1

As has been described so far the previously proposed secondary heat exchanger extracts only a small percentage of the latent heat from condensing steam and the purpose of the present invention is to substantially increase the extraction rate of this latent heat. Reference is hereby made to the drawings and related description in UK Patent Application No. 0107963.1 for details of the design and functionality of the previously proposed secondary heat exchanger.

In the embodiment shown in FIGS. 1 and 2, direct flue gas cooling is achieved if condensate water is pumped from a reservoir 10 via a pump 12 and pipe 14 to a perforated pipe 16 located above, centred and parallel to the upper face of a core 18, through which water at 60° C. or less flows as it returns from a heating system to the main boiler heat exchanger. The design of core 18 may be such as is described for the secondary heat exchanger in UK 0107963.1. Water from 16 will flow down the external surfaces of the core as a film or sheet of water eventually to fall into and entirely fill by overflowing a shallow tray 20 which may comprise the reservoir 10, but is more preferably the top one of a plurality of similar trays 22, 24, 26 each of which overflows to feed the one below and the lowest of which feeds the reservoir 10. The core 18, perforated pipe 16, trays 20 to 26 and reservoir 10 are located within or formed by a housing 28.

In parallel the flue gases from the main boiler are fed into the housing 28 via pipe 80 to mix with the water escaping from 16 and to travel therewith as it passes over the core 18 and via trays 20 to 26 to the reservoir 10.

As the film of water cascades down the outside of the core, the hot gases will be cooled by contact therewith, and will continue to be cooled as they traverse the surface of the water in the trays 20 et seq. In cooling, steam in the flue gases will be cooled below the steam/water transition temperature and in doing so its latent heat will transfer into the water in the trays, and the condensing steam will also be absorbed into the water. Thus simultaneously the water temperature and volume will increase. The water flows into the reservoir 10 from which it will be drawn by the pump 12 and recirculated via the perforated pipe 16 to be cooled as it again makes contact with the surface of the core 18 which is internally maintained at 60° C. by the return flow to the main boiler. As the process proceeds, the increase in volume of the circulating condensate can be drawn off by overflow pipe 32, and this excess water could amount to several litres per hour depending on boiler heat output.

The path of the hot gases from inlet 30 to the final flue outlet 31 (see FIG. 1) is denoted by arrows such as 33, 35.

If significant condensation is to be achieved the temperature of the cascading water needs to be below the dew point of the water vapour in the flue gases. Typically this temperature should be below 40° C., preferably below 30° C.

SECOND EMBODIMENT

In the arrangement shown in FIGS. 3-5 the core is replaced by five water filled trays 34, 26, 38, 40, 42, each tray being individually cooled by one or more heat exchange tubes 44, 46, 48, 50, 52, arranged to extend across each of the trays, and typically will be immersed by the water in each tray when the system is operating. This design is more compact than that of FIGS. 1, 2 and has a larger flue gas cooling area contained within any given size of housing 28. Water overflowing from 52 is collected in the well 54 at the bottom of the housing.

Hot gas entering the top of the housing from the pipe 30 encounters the water surface of the first tray 34. This water is cooled as before by the water returning to the main boiler. As the hot gases pass over and under the trays, the gases are cooled first by contact with the water surface and then with the underside of the tray.

It is possible that insufficient cooling of the gases will occur by tray 34 to condense steam present in the gases into tray 34. In that event a small pump (not shown) can be provided to circulate condensate from the well 54 to tray 34, from which it will overflow and, by cascade, all the trays will be constantly replenished.

The cooling tubes 44 et seq. of FIG. 5 may be of circular or square or rectangular cross section. Thus FIG. 3 shows a circular section pipe and FIG. 4 a rectangular cross section pipe. By careful selection of height and width dimensions of the latter, an optimum pipe surface area can be achieved for any given size of tray, given that the larger the proportion of the tray cross section occupied by a pipe the smaller will be the volume available for water in the tray.’

An overflow pipe 32 is shown in FIG. 5 showing how if water in the well 54 exceeds the height of the overflow pipe, water will leave 54 and can be recovered for re-use elsewhere or simply drained to waste.

In a complete system, the “excess” water can be simply run to waste, or be stored for use as low temperature hot water in a domestic, office or factory environment, or allowed to cool naturally to external ambient temperature for use as “cold” water, perhaps for irrigation or return to water reserves underground for subsequent recovery and use.

The passage of the hot gases from inlet pipe 30 to the final flue outlet 56 is denoted by a series of arrows such as 58, 60.

As with the FIGS. 1, 2 embodiment, the temperature of the cascading water should be below the dew point of the water in the flue gases, and in practice should be below 40° C., and preferably below 30° C. to achieve a high level of condensation of the steam content of the flue gases.

Comparison of Cooling Capabilities

The total cooling surface of the original core 18 is typically 3500 square centimetres.

Considering now the alternative arrangement of FIG. 5. If there are 5 trays in the FIG. 5 embodiment, each 11 cms wide and 52 cms long, then the total surface area of the trays is 2860 sq. cms.

The exhaust gases also traverse the water in the well. If the width of the casing is greater than the width of the trays by 1 cm, the water surface area in the well is 12×52=624 sq. cms.

Therefore the total gas/water interface is (2860+624)=3484 sq. cms.

The gas/metal interface is made up of the total surface area of the walls of the trays. If each tray is 11 cms wide by 3 cms high, the wall area per tray exposed to the passing exhaust gases is (3+3+11)×52=884 sq. cms.

If there are 5 trays the total metal surface area will be 5×884=4420 sq. cms.

The total surface available for cooling the gases is given by adding the areas of the gas/water and gas/metal interfaces.

Therefore the total surface area for cooling is 3484+4420=7904 sq. cms.

The water in the trays is of course itself cooled by the return flow to the boiler through the pipes, and if a small gap exists below each pipe (such as 44) the total external surface of each pipe will be in contact with water. If the rectangular cross-section pipes have a cross-section of 7×2 cms, the surface area of the 5 pipes (each 52 cms long) will be 5×(7+7+2+2)×52=4680 sq cms.

The area of 7904 sq. cms available in a FIG. 5 arrangement compares very favourably with the 3500 sq. cms of exhaust gas cooling surface available in the original core 18 of FIGS. 1 and 2. Furthermore this larger area can be packed into a height of less than 30 cms, typically 28 cms.

As with the earlier embodiments, the temperature of the water passing through the heat exchange tubes (or pipes) should be below the dew point of the water vapour content of the flue gases, typically below 40° C. and preferably below 30° C.

Conventionally hot water leaving the boiler for the radiators is at 72° C. and typically the flow rate through the radiators is adjusted so that the return water temperature is of the order of 60° C. It is possible to arrange matters so that this return water is at a lower temperature such as 30° C. or lower, so as to achieve useful condensation of the steam/water vapour in the flue gases. This will of course raise the temperature of the water before is passes to the boiler to be reheated, which is to advantage since it is normally considered desirable for the return water not to be too low in temperature.

Comparison with Other Fuels

The foregoing has assumed the fuel to be kerosene or the like. However similar advantages are obtained if the fuel is a gas such as natural gas, Propane or Butane. These gases all contain hydrogen as follows:— Natural gas 23.18% by weight Propane 17.98% by weight Butane  17.21% by weight.

The recoverable latent heat from these fuels will be directly proportional to the percentage of hydrogen present in the fuels.

Turning now to FIG. 6, the system shown will now be described in detail.

1. Commencing with the boiler 100, hot water at about 72° C. is pumped around a number of heating devices, such as conventional radiators 96. In the case of an ordinary heating system this hot water is returned directly to the boiler for re-heating and continuous recycling. As indicated above, the usual return temperature of the water is about 60° C. The fall in temperature of 12° C. relates to the heat given out by the radiators into the building. In the arrangement shown in FIG. 6 the 60° C. return water is directed via pipe 98 towards the upper core 102 of a secondary heat exchanger 104.

2. In accordance with the invention a small proportion of this return water is directed via a pipe 99 to the input of an additional heat exchange device 110. A valve 101 allows the proportion to be controlled. Typically 5% of the flow along 98 will be diverted along 99, leaving 95% to pass along pipe 103 to the upper core 102.

3. At the same time flue gases from the boiler, typically at a temperature in the range 150°-250° C., are led into the top of the secondary heat exchanger via duct 106 and are free to flow downward, over and around the external surface of the upper core 102. As this takes place the flue gases give up energy to the core 102. The gas temperature falls to a temperature in the range 75°-95° C., and the water flowing through core 102 rises in temperature, typically by a few degrees centigrade. This water, now typically at a temperature of 62° C. instead of 60° C., is returned to the main boiler 100 via pipe 108. It should be noted that this apparently small temperature rise should be seen in relation to the 12° C. temperature drop across the radiators mentioned above. This would give a probable boiler efficiency of about 92% if no other steps were taken to improve efficiency. In general there will be no condensation of water vapour in the flue gases at temperatures of 75° C. or higher, since dew point is most probably around 60° C.

4. The low flow of water along pipe 99 at approximately 60° C. will in a domestic installation be typically about ½ litre per minute. The additional exchange device may be a household radiator 110. To achieve maximum overall efficiency this radiator must be contained inside the building which is heated by the other radiators 96 for reasons which will be explained later. The radiator 110 may be an ordinary panel radiator with or without fins, and may be fan assisted, but should be of such a size and be positioned in the building so that water flowing therethrough at the rate governed by valve 101 (typically ½ litre per minute), and at a temperature of 60° C. on entry, will drop to a much lower temperature before it leaves the radiator. Typically an exit temperature of 35°-40° C. can be achieved in a domestic dwelling but an even lower temperature would be better still.

On leaving the radiator 110 the water, still flowing at about ½ litre per minute and at a temperature of say 35° C., is directed via pipe 112 to a further heat exchanger 114 (typically a so-called honeycomb air to water heat exchanger) located in the air intake to the burner of the boiler 100. The heat exchanger 114 is therefore subjected to an inflow of air, the temperature of which in the winter will typically lie in the range 0°-10° C., typically 5° C., and on very cold days may even be lower than 0° C.

The aim of heat exchanger 114 is to cool the water leaving the radiator 110 to as low a temperature as possible, while still keeping this water part of the constant volume of water contained within the closed system made up of the boiler 100 and the radiators 96.

This cool water from 114 is now directed into the lower heat exchanger core 116 via pipe 118.

The lower part of 104 comprises a reservoir 120 containing water 122 which surrounds and just covers the central core 116. The level is governed by a weir 124. Alternatively or in addition a syphon may be employed to maintain the level.

The water 122 is cooled by the water in 116, and in winter it is possible to lower it to a temperature which will be below the dew point of water vapour contained in the flue gases entering 104 via 106.

Water 122 from around 116 is drawn out by a pump 124 and delivered to a manifold 126 containing jet orifices providing a cascade of water for mixing with descending flue gases. A tortuous path may be presented to the water and gases by a plurality of horizontally spaced apart baffles 128.

After passing down and around core 102, the flue gases will now be at a temperature in the range 75° C.-95° C. and these gases are mixed with the water cascading from the manifold 126 and if provided around the baffles in 128. The gases are thereby cooled before exiting to the flue 130 and the area and number of the baffles 128 and volume of water flow from the manifold 126 are selected so as to achieve the desired temperature drop in the gases, prior to exit, so that the exit temperature is typically in the range 40° C.

In the case of space heating systems, boilers such as 100 will normally only be operated in winter, and in the UK the external winter average ambient temperature is 5° C. (about 41° F.).

It is at such times that the system should operate at the highest overall efficiency in terms of fuel/energy conversion and with the least CO₂/SO₂ loss to atmosphere. Of course, if the ambient temperature is much lower than 5° C., the probability is that total efficiency of the system will be even higher. If the ambient temperature is higher, the amount of energy needed to heat the building will be less and therefore the quantity of fuel burnt will be less and the quantities of CO² and SO² will be proportionately less.

The need to obtain cold water is to enable water vapour in the flue gases to be condensed so as to recover the latent heat energy in that “low temperature steam”.

It can be shown that, in the case of a boiler operating at a rated output of 80,000 BTUs per hour, the mass of air demanded by the burner is about 34 kilograms per hour. When this mass of air per hour is passed through the heat exchanger 114 at 5° C., water at 35° C. is found to be cooled typically to 15°-20° C., and the air entering the burner will itself be heated to a temperature typically in the range 20°-30° C.

In a domestic situation, the total area of the baffles is typically in the range of 1-2 square metres, and since the water will cool both faces of the baffles the total cooling area will be twice that, i.e. typically 2-4 square metres. Moreover, since the cooling of the gases is by direct contact with the cooled water, it is not difficult to ensure that the gas temperature falls to below the dew point of water vapour in the gas stream, so causing condensation and loss of heat from the vapour and simultaneous cooling of the hot gases.

In general the baffles 128 should be packed as tightly as possible in the space available so as to present a tortuous path for the exhaust gases and the water. In general the more baffles the better.

In the case of an 80,000 BTHu boiler, the expectation is that about 2 litres per hour of water will condense out from the flue gases, and a flue gas temperature of 30° C.-40° C. should be achieved.

Calculations show that the energy recovered from the condensation of water vapour to produce 2 litres of water per hour, is 4.5 mega joules which is equivalent to 1.3 kilowatts/hour. This energy will raise the temperature of the water 122 circulating around core 116 unless it is cooled, and it is for this reason that it is necessary to remove each hour slightly more than 1.3 kilowatts/hour from the ½ litre/minute water flow. This energy of approximately 1.3 kilowatts/hour is equivalent to about 4% of the energy provided by the radiators 96, where the water flow, and the flow and return temperatures are as specified above. Therefore if radiator 110 (which dissipates this heat in order to cool the return water to 35-40° C.) is located within the building which also contains the radiators, its heat output can be added to that from the radiators 96, so as to produce a theoretical efficiency of the order of 96% for the system taken as a whole.

In addition however the transfer of heat to the incoming air to the boiler can increase the energy available from the boiler. For an 80,000 BTHu boiler, this increase can be just over ½ kilowatt per hour. This means that less fuel is needed to generate the rated heat output from the boiler, and a slightly increased overall efficiency obtained. Typically the further increase in efficiency can be of the order of ½%.

Total system energy efficiency can therefore be of the order of 96.5%.

The only loss is the overflow of water from 124.

Sulphur dioxide can be removed as follows:

From published information, sulphur dioxide is very soluble in water. At NTP, a given volume of water will absorb 39 times that volume of sulphur dioxide. Thus if 30 litres of water per minute is cascading down the baffle plates at 128 this volume of water will readily absorb far more sulphur dioxide than could possibly be present in the volume of exhaust gases passing through and present in the baffle containing region of the heat exchanger housing 104 in the same period of time.

As the water vapour condenses it will increase the volume of the water 122 in the reservoir 120. This rising volume is controlled by weir 124. Water draining over the weir carries with it absorbed sulphur dioxide which can be neutralised by using the alkaline materials such as are used in domestic laundry facilities. Since the circulating water 122 will also become acidic, it may be necessary to remove this acidity by a neutralising cartridge 132 which may be in the form of a replaceable or rechargeable unit. The pipework 134, pump 124, manifold 126, baffles 128, core 116 and interior of the housing of the heat exchanger 104 should if possible be formed from or coated with a material not significantly chemically affected by sulphurous acid, or where appropriate be constructed in such a way as to be readily replaceable such as at annual servicing intervals.

Whether spaced apart baffles 128 are employed, or the exchanger relies on mixing the gases with fine water sprays, one thing is common to all variations, namely that the hot exhaust gases are cooled by direct contact with cold water, and the recovered sensible and latent heat of condensation thereby recovered from the flue gases, is incorporated into the heat supplied to the building and/or to the boiler heating radiators within the building so as to increase the overall efficiency of the installation.

The closed loop system described herein is aimed principally at the heating of one building such as a domestic dwelling or single office building. Nevertheless any circulating hot water heating system can be improved by the invention, which could be applied to a town or city district heating system, or to a multiple occupation high rise building or office block, provided in each case the system includes a boiler house from which hot water is circulated for heating and to which cooler water is returned. The CLW/HG procedure will function equally well, whether the heat output is 30 KW or 300,000 KW, the only difference being that of scale.

Thus with reference to FIG. 6(i), item 96 can be a set of radiators in one house, or all the radiators in all the houses in a district heating system.

In FIG. 6(ii) the heat exchanger is shown as having an upper heat exchange core 102 (which will be referred to as A) and a lower heat exchanger core 116 (which will be referred to as C) inside the one casing. The efficiency of these exchangers can be improved. Thus in the example shown there is one of A and one of C. In practice and for the sake of saving space, multiples of A or C are possible.

A preferred arrangement is shown in FIGS. 7 and 8 in which there are two A-units A1, A2, arranged in parallel and a single C-unit below. Additionally a problem has been noted with the layout as shown in FIG. 6(ii).

FIG. 7 shows the nature of the problem. Note first the hot exhaust gas inflow M. This hot gas does not uniformly spread over and around the enclosure forming core A, and will probably never reach corners x and y of the enclosure.

Secondly the water flow in and out of core A will tend to take the shorter path as shown by P. It is therefore probable that there will be little if any water flow to region Z of the enclosure forming core A. Even if water were introduced and removed at opposite ends of the enclosure, the result would be similar, with some regions not seeing any water flow at all.

The core C in the lower heat exchanger, which is surrounded by water, suffers a similar problem, in that internal flow of water will tend not to reach region ZZ of the enclosure forming core C.

The net result is a marked reduction of effective heat exchange area and therefore of total efficiency.

The solution is shown in FIG. 8.

Firstly curved baffle plates E, F deflect the incoming gases causing them to change direction left and right. This causes exhaust gas to flow more evenly over and around the two enclosures of core A, thus improving its contact with regions x and y of each enclosure.

Secondly, internal baffles J and K are provided in enclosures A and C, five in A and three in C.

The baffles create a tortuous zig-zag path for water through the enclosures, and by connecting the baffles to the walls of the enclosures water circulating through A and C is forced into all the corners, thus reducing or eliminating the low efficiency areas at Z and ZZ identified in FIG. 7.

By making them of thermally conductive material the effective surface area of material in contact with the water is increased if the baffles also make good thermally transmittive contact with the walls of the enclosures forming the cores of the heat exchangers.

By making the baffles of relatively rigid material such as a thermally conductive metal, they will not only create the tortuous path and increase the effective surface area, but will also internally brace the enclosures of cores A and C.

In FIG. 10 a typical domestic boiler 210 is shown having a burner 212 in combustion chamber supplied with fuel by a fuel supply pipe 214, and from an air intake 216. The boiler includes a pump 218 and a main heat exchanger 220 through which water in use is circulated by the pump 218. The water is heated by the hot product of combustion rising from the burning fuel.

Hot water leaves the main heat exchanger 220 via hot water outlet 222 and pipe 224, and after circulating through radiators 226 the water is returned via pipe 228 to an inlet 230.

The hot gases leave the upper end of the boiler casing through outlet 232, and via duct 234 pass to the flue gas inlet 236 at the upper end of a secondary composite heat exchange device, the casing for which is generally designated 238.

The casing 238 is divided internally by a wall 240 and in the upper compartment 242 is located a coiled tube 244 around which the gases entering at 236 flow downwardly through the compartment to exit via outlet 246 and pipe 248, so as to pass via an inlet 250 into a manifold 252 located approximately midway up the lower part 254 of the casing 238 below the wall 240.

Water flows through the coiled tube 244 as will be described later, to cool the hot gases as they migrate through upper compartment 242, and the water in turn is heated.

The lower compartment 254 includes upper and lower regions 256, 258 separated by the manifold 252 into which the gases pass from duct 248, and from which the gases leave in a generally upward direction.

In the upper region 256 is an array of spaced apart baffle plates 260 which force the gases to follow a tortuous path as they rise upwardly through the region 256 to exit via outlet 262 and pass via duct 264 to a flue or chimney (not shown).

Just below the wall 240 is a spray bar assembly 266 to which cold water is supplied via a pipe 268. Water leaving the spray bar 266 forms a cascade of water over the baffles 260 which travels down the region 256 in the opposite sense to the hot gases rising up therethrough, so as to mix with, and cool the gases.

After reaching the bottom of the region 256 containing the baffles 260, the water drains down into the lower region 258 which contains a coiled tube 270. In use water is supplied via pipe 272 to an inlet 274 to the tube 270 and flows through the tube 270 through an outlet 276 to a pipe 278. The latter conveys water to an inlet 280 to the coiled tube 244 in the upper region 242, so that after traversing the lower coil 270 water passes through the upper coil 244 in the upper region 242, and leaves via outlet 277 and pipe 279 to return to pipe 228 and thereby via pump 218 to the heat exchanger 220 in the boiler 210.

An outlet 282 at the bottom of the lower region 258 allows water which has collected in the lower region to pass via a pipe 284 to a pump 286 to be pumped via pipe 268 to the spray bar 266. The lower region 258 can be thought of as a reservoir.

As water vapour in the hot gases condenses, this also drains into the water in the lower region and this will raise the level of the water in the reservoir 258. An overflow 289 preferably including a siphon, prevents the level of water in 258 exceeding a design level, and allows excess water to be drained off.

A valve 290 is positioned in the return feed pipe 228 upstream of where pipe 279 enters pipe 228. If partially closed, valve 290 will create a back pressure upstream of the valve. A bleed pipe 292 communicates with the pipe 228 upstream of valve 290 and depending on the setting of the valve, more or less water will be diverted from 228 into 292.

Pipe 292 feeds a radiator 294 located in a cool part of the building (not shown).

Water from 294 flows along a pipe 296 to a coil 298 in a cold water tank 300 usually located in the roof space of a building, which is supplied with cold water from a water main (not shown) for supplying water to a hot water cylinder (not shown) to be heated either by an immersion heater or by a heat exchanger in parallel with the radiators (or both). After passing through the coil 298 the water will be cooled and the now cooled water flows via pipe 272 to the inlet 274 to the coil 270.

Further cooling is achieved by diverting some of the water from pump 286 via a valve 310 and pipe 312 to an inlet 314 to the heat exchanger 316 located in an air duct 318 which serves as an air inlet to the building (not shown). Water from the heat exchanger 316 passes via outlet 320 and pipe 322 to a low level inlet 324 to the water reservoir 258.

The air flow into and through the duct 325 may be fan assisted as by a fan 326. On cold days in winter, the incoming air temperature in the UK could be below 0° C. thereby causing a significant cooling of the water flowing through 316, but in turn the air is warmed perhaps to in excess of 20° C.

A control system may be provided as shown at 328 which receives signals from sensors (not shown) sensing air temperature, water temperature, air flow and water flow rates, to provide control signals for operating valves such as 290 and 310 and/or fan speeds and/or pump speeds.

Air required for combustion may instead or in addition be warmed by a heat exchanger 330 in the air duct 216. Water which is to pass though the heat exchanger 330 may be obtained by diverting water from pipe 292 along a pipe 332 to supply an inlet 334 to the heat exchanger 330. After traversing the latter the now cooled water leaves via outlet 336 to flow via pipe 338 to pipe 292 or pipe 296. A valve (not shown) may be included in the pipe 332 or 338 to control the flow rate through 330.

Where all the water in 292 is to flow through 330 before it passes to 294, the pipe 292 must be broken at X so that the diverted water in 292 is forced to flow along 332, through 330 and then via 338 to the inlet of radiator 294.

Where some of the water flow in 292 is to be diverted via 330, then a valve (not shown) may be inserted in pipe 292 downstream of the tapping (not shown) feeding pipe 332 and upstream of where pipe 338 joins the pipe 292 ahead of radiator 294. Adjusting the valve (not shown) allows more or less water to flow through 330.

Where 330 is to be in parallel with 294, then pipe 332 returns the water from 330 to pipe 296.

In order to warm the air supply for combustion of the fuel supplied to the burner 212, the air duct 216 may be supplied with air that has been warmed by being passed around the casing 238 of the secondary heat exchanger. This arrangement is shown in FIG. 2 in which the casing 238 is shown housed within (but spaced from) an external housing 304. The latter creates a closed space around 238 having an air inlet 306 and outlet 308. The latter communicates with the inlet of duct 216. Heat from the casing 238 warms the air flowing between 306 and 308 thereby raising the temperature of the air to be employed in the combustion of the fuel by the burner 212.

FIG. 12 shows an alternative cold water reservoir 300′ which can be used in place of that shown at 300 in FIG. 10A. In the alternative reservoir the heat exchanger is outside the tank 300′ and is denoted by the dotted outline 340. Water is drawn from the tank 100′ via outlet 342 and pipe 344 by a pump 346 to be a pumped through a pipe 347 to a sleeve 348. The latter surrounds, and is spaced from, an inner pipe 350, and serves to return water to the tank via a pipe 352 and inlet connection 354, after traversing the length of the sleeve.

Water to be cooled by the cold water passing through the sleeve 340 is obtained from pipe 296 (of FIG. 10A) which is connected to the pipe 350 at 356. After being cooled, it passes out of the pipe 350 via outlet connection 358 and enters pipe 272 (of FIG. 10A) to pass to coil 270 via the inlet connection 274.

Also shown in FIG. 12 is a cold water feed pipe 360 for supplying water to the tank 300′, a valve 362 of conventional design which controls the inflow of water into the tank and a hollow ball 364 which floats on the water and is carried at the end of a hinged arm 366. This operates in known manner to close the valve 362 when the water level 368 in the tank raises the ball 364 to a predetermined height.

Power for operating the pump 346 is obtained from the control system 328, and although not shown, a warning device (which may be either audible or visual or both) is provided to indicate if the pump is failing to pump water around the sleeve 348.

Typically the pipe 350 is formed from copper or other good thermally conducting material. The outer sleeve may also be formed from copper or the like, or more preferably from a plastics material. In either event, but especially if formed from copper or the like, the outer sleeve is preferably encased in a thermally insulating jacket or enclosure as shown in dotted outline at 340.

As shown in FIG. 12 the sleeve 348 is closed at its two ends at 370, 372 and the smaller diameter pipe 350 extends in a sealing manner through the sleeve ends 370, 372 respectively.

For given diameters, the length of the pipe 350 and sleeve 348 will determine the area of the heat transfer surface and if the pipe is of the order of 5-10 mm diameter and the sleeve is in the range 10-20 mm diameter, then the length of the sleeve will probably need to be of the order of 5 to 6 metres. The actual length can be determined by experiment. For convenience the combination of sleeve and pipe are therefore coiled as shown in dotted outline in FIG. 12.

Although not shown, the pipe 344, pipe 347 and pipe 352 may also be encased in thermally insulating material, as may also the pump if desired. Alternatively some or all of these pipes and the pump may be situated within the enclosure 140 and thereby be insulated.

It is a preferred feature of the heat exchanger of FIG. 12 that the pump 346 operates to cause water to flow in the direction shown by the arrow 374 so that the water flow through the sleeve is opposite in direction to that through the pipe 350. This contra flow has been found to increase the transfer of heat by the device relative to what is obtained if the water flow in the sleeve and pipe are in the same direction. The water flow back to the tank 300′ is denoted by arrow 375.

Also shown in FIG. 12 is a temperature sensitive switch 376 including a temperature sensor 378, which is connected to the control system 228 to shut off the supply of power to the pump 346 if the temperature of the water in 300′ rises above the predetermined valve.

An alternative contra flow heat exchanger for placing inside the cold water tank is shown in FIG. 13.

Superficially the system is similar to that of FIG. 12 but there is one important difference.

Since the FIG. 13 exchanger is inside the tank the outer surface of the outer tube is in contact with the cold water and the device therefore has about 150% more effective heat exchange surface.

In FIG. 13 water is pumped from the tank 180 into and along inner tube 384 by a submersible pump 384.

After traversing 380 the pumped water exits back into the water in the tank 380.

Water from the divert system is caused to pass along the annular passage 386 between inner pipe 382 and outer pipe 388, in the opposite direction.

The area of outer tube which is exposed to the tank water is proportional to the outer tube diameter, whereas the area effected by the pumped contra flow, is proportioned to the inner tube diameter. Typically the inner tube diameter is 8 mm and the outer tube diameter is 12 mm.

If the tubes 382, 388 are each 1 meter long, then the surface area of inner tube=100×0.8×3.14=251.2 cm² and that of the outer tube=100×1.2×3.14=376.8 cm².

This results in a total heat exchange area equal to 628 cm².

Typically the water in the inner tube 382 from the cold water in the tank will be at a temperature of the order of 8° C., and it moves along in the arrowed direction, cooling as it does so, the wall of the inner tube 382.

The diverted water which flows through the annular region 386, moves in the opposite direction, and is cooled by the water flowing through 382. However the outer tube 388 is also exposed to the cold tank water, so the divert water is cooled on one face by the contra flow of cold water in 382 and simultaneously gives up its energy quite efficiently to the cold water in the tank through the wall of outer tube 386.

If the annular gap is small, eg. of the order of 2 mm, there is only a small volume for the 0.5-1.00 litre per minute flow of the diverted water to occupy as it traverses the annular region 186.

In practice the pipes 382, 388 will need to be a few metres long, for example 5 metres, and can be accommodated in a domestic cold water tank by being coiled. The assembly may be attached to, and the flow and return pipes 390, 392 for the divert water can also extend through, a tank lid, such as shown at 394. In addition, the ball valve and cold water feed as described with reference to FIG. 12, may also be attached to the tank lid 394.

If after being coiled, the exit from the inner tube 382 is near the top of the water in the tank, a diffuser such as 396 is fitted over the exit to break up the flow and prevent turbulance. 

1. A method of recovering latent heat from water vapour in the flue gases from a hydrocarbon fuel-burning boiler adapted to heat water to be circulated through radiators to heat a building, in which hot flue gases from the boiler are cooled first by a heat exchanger through which water returning from the radiators is caused to flow and then by a second heat exchanger through which water at a temperature below the dew point of the water vapour in the flue gases is caused to flow in direct contact with the flue gases, so that not only is heat extracted from the latter but soluble products of combustion (such as SO₂) are dissolved in the water and are thereby separated from the flue gases before they exit to atmosphere, and wherein before the water returning from the radiators is supplied to the first heat exchanger a small proportion of the returning water is diverted through an additional heat exchange device located in a relatively cool region in the building being heated, and thereafter the diverted water is further cooled as it passes through an air to water heat exchanger through which air which is to support combustion in the burner of the boiler passes en route to the combustion chamber, such that the exit water temperature therefrom will be in the range of 20°-40° C., so as to be below the said dew point and the exiting water is supplied to cool the water in the second heat exchanger.
 2. A method as claimed in claim 1 wherein the second heat exchanger includes a subsidiary water to water heat exchanger through which the said exiting water flows to cool water which flows around a closed circuit defined by the second heat exchanger, and after passing through the subsidiary heat exchanger the water is returned to the boiler.
 3. A boiler for supplying hot water to radiators in a building having first and second heat exchange means for cooling the exhaust gases through the first of which water returning from the radiators is caused to flow before returning to the boiler, which includes a water path by which a fraction of the water returning from the radiators to the first heat exchanger is first caused to flow direct to an additional heat exchange device instead of to the first heat exchanger, after passing therethrough to pass through an air to water heat exchanger in an air inlet to the boiler supplying air to support combustion therein, thereafter to pass through a water to water heat exchanger adapted to cool water circulating in the said second heat exchanger, and lastly to return the water to the water inlet to the boiler, where it is mixed with water passing thereto from the first heat exchanger.
 4. A method of heating a building in which a water heating boiler which in use burns a hydrocarbon fuel in a combustion chamber from which hot products of combustion leave to vent to atmosphere via a flue, and which provides a source of hot water for circulating through radiators in the building, and a secondary condensing heat exchange means operates to lower the temperature of the products of combustion below the dew point of water vapour therein thereby to gain the latent heat of evaporation of the water content thereof to the flue, and in which there is an air inlet to the building, comprising the step of warming air entering the building by a heat exchanger in the air inlet through which water is caused to flow which in use is heated in the secondary heat exchange means using heat from the hot products of combustion, thereby to transfer heat therefrom to air entering the building.
 5. A method as claimed in claim 4 wherein the air inlet to the building also comprises an air intake for supplying air to the combustion chamber of the boiler, so that after passing through or around the heat exchanger in the inlet some of the warmed air goes to the combustion chamber in the boiler to support combustion and the remainder of the warmed air is diverted into the building, to directly warm the latter.
 6. A method as claimed in claim 4 wherein the secondary condensing heat exchange means comprises two separate heat exchangers, a first in which the hot flue gases are cooled by water flowing therethrough to a temperature which is above the dew point temperature and a second through which the previously cooled flue gases pass and in which they are cooled to below the dew point temperature before escaping to atmosphere, and in which the second heat exchanger includes primary and secondary water circuits, water flowing around the secondary circuit being cooled by water flowing through the primary circuit, the secondary circuit water being in contact with the flue gases to cool them below the dew point, and water from the secondary circuit is caused to pass through the heat exchanger in the air inlet, to cool the secondary circuit water and warm the incoming air, before being returned to rejoin the water flowing around the secondary circuit, and water which is to flow through the primary circuit is obtained by diverting some of the water returning from the radiators in the building to the boiler, and this diverted water is cooled by flowing through at least one further heat exchange means before being supplied to an inlet to the primary circuit.
 7. A method as claimed in claim 4 wherein the secondary condensing heat exchange means comprises two separate heat exchangers, a first in which the hot flue gases are cooled by water flowing therethrough to a temperature which is above the dew point temperature and a second through which the previously cooled flue gases pass and in which they are cooled to below the dew point temperature before escaping to atmosphere, and in which the second heat exchanger includes primary and secondary water circuits, water flowing around the secondary circuit being cooled by water flowing through the primary circuit, the secondary circuit water being in contact with the flue gases to cool them below the dew point, and water from the secondary circuit is caused to pass through the heat exchanger in the air inlet, to cool the secondary circuit water and warm the incoming air, before being returned to rejoin the water flowing around the secondary circuit, and water which is to flow through the primary circuit is obtained by diverting some of the water returning from the radiators in the building to the boiler, and this diverted water is cooled by flowing through at least one further heat exchange means before being supplied to an inlet to the primary circuit, wherein the said at least one further heat exchange means comprises a radiator located within a cool region of the building.
 8. A method as claimed in claim 4 wherein the secondary condensing heat exchange means comprises two separate heat exchangers, a first in which the hot flue gases are cooled by water flowing therethrough to a temperature which is above the dew point temperature and a second through which the previously cooled flue gases pass and in which they are cooled to below the dew point temperature before escaping to atmosphere, and in which the second heat exchanger includes primary and secondary water circuits, water flowing around the secondary circuit being cooled by water flowing through the primary circuit, the secondary circuit water being in contact with the flue gases to cool them below the dew point, and water from the secondary circuit is caused to pass through the heat exchanger in the air inlet, to cool the secondary circuit water and warm the incoming air, before being returned to rejoin the water flowing around the secondary circuit, and water which is to flow through the primary circuit is obtained by diverting some of the water returning from the radiators in the building to the boiler, and this diverted water is cooled by flowing through at least one further heat exchange means before being supplied to an inlet to the primary circuit, wherein the building includes a reservoir of cold water, a heated water cylinder and taps to which water is supplied therefrom, and the further heat exchange means comprises a further heat exchanger associated with the cold water reservoir, through which the diverted water is caused to flow before it is supplied to the inlet to the said primary circuit.
 9. A method as claimed in claim 4 wherein the secondary condensing heat exchange means comprises two separate heat exchangers, a first in which the hot flue gases are cooled by water flowing therethrough to a temperature which is above the dew point temperature and a second through which the previously cooled flue gases pass and in which they are cooled to below the dew point temperature before escaping to atmosphere, and in which the second heat exchanger includes primary and secondary water circuits, water flowing around the secondary circuit being cooled by water flowing through the primary circuit, the secondary circuit water being in contact with the flue gases to cool them below the dew point, and water from the secondary circuit is caused to pass through the heat exchanger in the air inlet, to cool the secondary circuit water and warm the incoming air, before being returned to rejoin the water flowing around the secondary circuit, and water which is to flow through the primary circuit is obtained by diverting some of the water returning from the radiators in the building to the boiler, and this diverted water is cooled by flowing through at least one further heat exchange means before being supplied to an inlet to the primary circuit, and wherein the diverted water returning from the radiators is forced to pass through a further heat exchanger in an air inlet to the boiler combustion chamber.
 10. A method as claimed in claim 4 wherein the secondary condensing heat exchange means comprises two separate heat exchangers, a first in which the hot flue gases are cooled by water flowing therethrough to a temperature which is above the dew point temperature and a second through which the previously cooled flue gases pass and in which they are cooled to below the dew point temperature before escaping to atmosphere, and in which the second heat exchanger includes primary and secondary water circuits, water flowing around the secondary circuit being cooled by water flowing through the primary circuit, the secondary circuit water being in contact with the flue gases to cool them below the dew point, and water from the secondary circuit is caused to pass through the heat exchanger in the air inlet, to cool the secondary circuit water and warm the incoming air, before being returned to rejoin the water flowing around the secondary circuit, and water which is to flow through the primary circuit is obtained by diverting some of the water returning from the radiators in the building to the boiler, and this diverted water is cooled by flowing through at least one further heat exchange means before being supplied to an inlet to the primary circuit, and wherein a pump operates to pump secondary circuit water through the second heat exchanger and through the building air inlet heat exchanger and wherein only some of the secondary circuit water is circulated through the air inlet heat exchanger before it returns to the secondary circuit, and the remainder of the secondary circuit water is simply circulated around the secondary circuit by the pump.
 11. A method as claimed in claim 4 wherein the secondary condensing heat exchange means comprises two separate heat exchangers, a first in which the hot flue gases are cooled by water flowing therethrough to a temperature which is above the dew point temperature and a second through which the previously cooled flue gases pass and in which they are cooled to below the dew point temperature before escaping to atmosphere, and in which the second heat exchanger includes primary and secondary water circuits, water flowing around the secondary circuit being cooled by water flowing through the primary circuit, the secondary circuit water being in contact with the flue gases to cool them below the dew point, and water from the secondary circuit is caused to pass through the heat exchanger in the air inlet, to cool the secondary circuit water and warm the incoming air, before being returned to rejoin the water flowing around the secondary circuit, and water which is to flow through the primary circuit is obtained by diverting some of the water returning from the radiators in the building to the boiler, and this diverted water is cooled by flowing through at least one further heat exchange means before being supplied to an inlet to the primary circuit, and wherein a further pump is provided which in use pumps the diverted water through the further heat exchange means and through the primary circuit, to allow the flow rate of the diverted returning water to be controlled.
 12. A secondary heat exchange means for recovering latent heat of evaporation from hot products of combustion from a water boiler which in use burns a hydrocarbon fuel in a combustion chamber to provide a source of hot water for circulating through radiators in the building comprising a closed housing divided internally into two compartments, an upper compartment containing a first heat exchanger, a flue gas inlet at its upper end, and a flue gas outlet at its lower end, and a lower compartment having upper and lower regions, a second heat exchanger forming the said primary circuit and located in the lower region thereof with water inlet and outlet connections thereto, a plurality of baffles defining a tortuous path between the top and bottom of the upper region of the lower compartment, duct means communicating between the flue gas outlet of the upper compartment and a flue gas inlet to the lower compartment for conveying flue gases between the two compartments, a flue gas outlet conveying flue gases from the lower compartment to atmosphere, pipe means for conveying water from a water outlet at the bottom of the lower region of the lower compartment to an inlet to a water spray means at or near the top of the upper region of the lower compartment, so that in use, water cascades downwardly over and around the baffles and is in contact with flue gases following the tortuous path around the baffles, to comprise said secondary circuit, and further pipe means for conveying water from an outlet of the second heat exchanger to an inlet of the first heat exchanger, so that in use the water flowing through the latter is warmed before it leaves the secondary heat exchange means to return to the boiler.
 13. A central heating boiler in combination with secondary condensing heat exchange means as claimed in claim 12 when forming part of a heating system for a building.
 14. A building when heated by a heating system operating as claimed in claim 4 to which heat is supplied from the combustion process by means of at least one of a plurality of different heat exchange processes, wherein air entering the building is heated, water which is supplied to radiator heat exchangers in the building is heated, cold water stored in the building is heated, and heat is gained from hot gaseous products of combustion by cooling them to a temperature below the dew point of water vapour therein before being exhausted to atmosphere, the heat thereby given up by the hot products of combustion and the latent heat of vaporisation of their water content also being employed to heat water and/or air employed in heating the building.
 15. A heating system for a building in which a hydrocarbon fuel is burnt in a combustion chamber in a boiler to heat water for circulating through radiators in the building, which produces hot gaseous products of combustion, and in which heat is exchanged between those products of combustion and secondary water which is cooled below the dew point temperature of water vapour in the products of combustion by primary water diverted from water returning from the radiators to the boiler, to achieve condensation of the hot exhaust gas water vapour content, wherein the diverted primary water is itself cooled to below the said dew point temperature and in use serves to cool the secondary water, and before the diverted primary water returns to the boiler, heat is exchanged between it and the hot gaseous products of combustion as a first step in the cooling of the latter and to raise the temperature of the primary water before it is remixed with the water returning to the boiler from the radiators.
 16. A building and heating system therefor which comprises a water heating boiler which in use burns a hydrocarbon fuel in a combustion chamber which produces hot products of combustion which vent to atmosphere via a flue and which provides a source of hot water, pump means for circulating the hot water through radiators in the building, a secondary condensing heat exchange means which in use operates to lower the temperature of the flue gases below the dew point of water vapour therein thereby to gain a latent heat of evaporation therefrom before they pass to a flue, and in which the building includes an inlet through which air enters the building in which a heat exchanger is located through which in use water which has been heated in the secondary heat exchange means is caused to flow, thereby to transfer heat from the latter to air entering the building, to warm the latter.
 17. A building and heating system as claimed in claim 16 wherein the air inlet to the building also comprises an air intake for supplying air to the combustion chamber of the boiler so that after passing through or around the heat exchanger in the air inlet some of the warmed air goes to the combustion chamber in the boiler to support combustion and the remainder of the warmed air is diverted into the building, to directly warm the latter.
 18. A building and heating system as claimed in claim 16 wherein an air inlet to the boiler combustion chamber is separate from the air inlet to the building and a heat exchanger is associated with each of the air inlets, and in use heated water is caused to flow through both heat exchangers.
 19. A building and heating system as claimed in claim 16 wherein the secondary condensing heat exchange means comprises two separate heat exchangers, a first in which the hot flue gases are cooled by water flowing therethrough to a temperature which is above the dew point temperature and a second through which the previously cooled flue gases pass and in which they are cooled to below the dew point temperature before escaping to atmosphere, and in which the second heat exchanger includes primary and secondary water circuits, water flowing around the secondary circuit in use being cooled by water flowing through the primary circuit, wherein the secondary circuit water is in contact with the flue gases to cool them below the dew point, and wherein water from the secondary circuit in use is caused to pass through the heat exchanger in the air inlet, to warm the incoming air and cool the secondary circuit water, before it returns to the secondary circuit.
 20. A building and heating system as claimed in claim 16 wherein the secondary condensing heat exchange means comprises two separate heat exchangers, a first in which the hot flue gases are cooled by water flowing therethrough to a temperature which is above the dew point temperature and a second through which the previously cooled flue gases pass and in which they are cooled to below the dew point temperature before escaping to atmosphere, and in which the second heat exchanger includes primary and secondary water circuits, water flowing around the secondary circuit in use being cooled by water flowing through the primary circuit, wherein the secondary circuit water is in contact with the flue gases to cool them below the dew point, and wherein water from the secondary circuit in use is caused to pass through the heat exchanger in the air inlet, to warm the incoming air and cool the secondary circuit water, before it returns to the secondary circuit and further comprising diverting means whereby in use some of the water returning from the radiators in the building to the boiler, is diverted through at least one further heat exchange means to be cooled therein before being supplied to an inlet to the primary circuit.
 21. A building and heating system as claimed in claim 16 wherein the secondary condensing heat exchange means comprises two separate heat exchangers, a first in which the hot flue gases are cooled by water flowing therethrough to a temperature which is above the dew point temperature and a second through which the previously cooled flue gases pass and in which they are cooled to below the dew point temperature before escaping to atmosphere, and in which the second heat exchanger includes primary and secondary water circuits, water flowing around the secondary circuit in use being cooled by water flowing through the primary circuit, wherein the secondary circuit water is in contact with the flue gases to cool them below the dew point, and wherein water from the secondary circuit in use is caused to pass through the heat exchanger in the air inlet, to warm the incoming air and cool the secondary circuit water, before it returns to the secondary circuit and further comprising diverting means whereby in use some of the water returning from the radiators in the building to the boiler, is diverted through at least one further heat exchange means to be cooled therein before being supplied to an inlet to the primary circuit and wherein the said at least one further heat exchange means comprises a radiator located within a cool region of the building.
 22. A building and heating system as claimed in claim 16 wherein the secondary condensing heat exchange means comprises two separate heat exchangers, a first in which the hot flue gases are cooled by water flowing therethrough to a temperature which is above the dew point temperature and a second through which the previously cooled flue gases pass and in which they are cooled to below the dew point temperature before escaping to atmosphere, and in which the second heat exchanger includes primary and secondary water circuits, water flowing around the secondary circuit in use being cooled by water flowing through the primary circuit, wherein the secondary circuit water is in contact with the flue gases to cool them below the dew point, and wherein water from the secondary circuit in use is caused to pass through the heat exchanger in the air inlet, to warm the incoming air and cool the secondary circuit water, before it returns to the secondary circuit and further comprising diverting means whereby in use some of the water returning from the radiators in the building to the boiler, is diverted through at least one further heat exchange means to be cooled therein before being supplied to an inlet to the primary circuit and wherein the building includes a reservoir of cold water, a heated water cylinder to which water from the cold water reservoir is supplied, and taps to which water is supplied from the cylinder, and the said further heat exchange means is a further heat exchanger associated with the cold water reservoir, through which the diverted water is caused to flow before it is supplied to the inlet to the said primary circuit.
 23. A building and heating system as claimed in claim 16 wherein the secondary condensing heat exchange means comprises two separate heat exchangers, a first in which the hot flue gases are cooled by water flowing therethrough to a temperature which is above the dew point temperature and a second through which the previously cooled flue gases pass and in which they are cooled to below the dew point temperature before escaping to atmosphere, and in which the second heat exchanger includes primary and secondary water circuits, water flowing around the secondary circuit in use being cooled by water flowing through the primary circuit, wherein the secondary circuit water is in contact with the flue gases to cool them below the dew point, and wherein water from the secondary circuit in use is caused to pass through the heat exchanger in the air inlet, to warm the incoming air and cool the secondary circuit water, before it returns to the secondary circuit and further comprising diverting means whereby in use some of the water returning from the radiators in the building to the boiler, is diverted through at least one further heat exchange means to be cooled therein before being supplied to an inlet to the primary circuit and wherein the building includes a reservoir of cold water, a heated water cylinder to which water from the cold water reservoir is supplied, and taps to which water is supplied from the cylinder, and the said further heat exchange means is a further heat exchanger associated with the cold water reservoir, through which the diverted water is caused to flow before it is supplied to the inlet to the said primary circuit and wherein the said further heat exchanger is a contra flow heat exchanger in which there are two water paths which are essentially parallel and in use the flow of water to be warmed along one path is opposite to the direction of the flow of water which is to do the warming and which flows along the other path.
 24. A building and heating system as claimed in claim 16 wherein the secondary condensing heat exchange means comprises two separate heat exchangers, a first in which the hot flue gases are cooled by water flowing therethrough to a temperature which is above the dew point temperature and a second through which the previously cooled flue gases pass and in which they are cooled to below the dew point temperature before escaping to atmosphere, and in which the second heat exchanger includes primary and secondary water circuits, water flowing around the secondary circuit in use being cooled by water flowing through the primary circuit, wherein the secondary circuit water is in contact with the flue gases to cool them below the dew point, and wherein water from the secondary circuit in use is caused to pass through the heat exchanger in the air inlet, to warm the incoming air and cool the secondary circuit water, before it returns to the secondary circuit and further comprising diverting means whereby in use some of the water returning from the radiators in the building to the boiler, is diverted through at least one further heat exchange means to be cooled therein before being supplied to an inlet to the primary circuit and wherein the building includes a reservoir of cold water, a heated water cylinder to which water from the cold water reservoir is supplied and taps to which water is supplied from the cylinder, and the said further heat exchange means is a further heat exchanger associated with the cold water reservoir, through which the diverted water is caused to flow before it is supplied to the inlet to the said primary circuit and wherein the diverted water is first caused to flow through a radiator located in a cool region of the building and thereafter through the heat exchanger associated with the cold water reservoir.
 25. A building and heating system as claimed in claim 16 wherein the secondary condensing heat exchange means comprises two separate heat exchangers, a first in which the hot flue gases are cooled by water flowing therethrough to a temperature which is above the dew point temperature and a second through which the previously cooled flue gases pass and in which they are cooled to below the dew point temperature before escaping to atmosphere, and in which the second heat exchanger includes primary and secondary water circuits, water flowing around the secondary circuit in use being cooled by water flowing through the primary circuit, wherein the secondary circuit water is in contact with the flue gases to cool them below the dew point, and wherein water from the secondary circuit in use is caused to pass through the heat exchanger in the air inlet, to warm the incoming air and cool the secondary circuit water, before it returns to the secondary circuit and further comprising diverting means whereby in use some of the water returning from the radiators in the building to the boiler, is diverted through at least one further heat exchange means to be cooled therein before being supplied to an inlet to the primary circuit and wherein the building includes a reservoir of cold water, a heated water cylinder to which water from the cold water reservoir is supplied, and taps to which water is supplied from the cylinder, and the said further heat exchange means is a further heat exchanger associated with the cold water reservoir, through which the diverted water is caused to flow before it is supplied to the inlet to the said primary circuit and wherein the further heat exchanger comprises two lengths of copper tube one inside the other and defining an annular water path between the two tubes which is closed at each end and includes a water inlet to the inner tube at one end and a water outlet therefrom at the other end, and a water inlet is provided to the annular water path near the outlet from the inner tube and an outlet from the annular path is provided near the inlet to the inner tube, with pipe connections for supplying the diverted water to the inlet and retrieving diverted water from the outlet of the annular path, and a pump is provided for circulating water from the reservoir through the inner pipe to re-enter the reservoir as it exits from the inner tube outlet, thereby to create a contra flow heat exchanger in which the two water paths are essentially parallel, so that in use the flow of water to be warmed along one path is opposite to the direction of the flow of water which is to do the warming and which flows along the other path.
 26. A building and heating system as claimed in claim 16 wherein the secondary condensing heat exchange means comprises two separate heat exchangers, a first in which the hot flue gases are cooled by water flowing therethrough to a temperature which is above the dew point temperature and a second through which the previously cooled flue gases pass and in which they are cooled to below the dew point temperature before escaping to atmosphere, and in which the second heat exchanger includes primary and secondary water circuits, water flowing around the secondary circuit in use being cooled by water flowing through the primary circuit, wherein the secondary circuit water is in contact with the flue gases to cool them below the dew point, and wherein water from the secondary circuit in use is caused to pass through the heat exchanger in the air inlet, to warm the incoming air and cool the secondary circuit water, before it returns to the secondary circuit and further comprising diverting means whereby in use some of the water returning from the radiators in the building to the boiler, is diverted through at least one further heat exchange means to be cooled therein before being supplied to an inlet to the primary circuit and further comprising a further heat exchanger in an air inlet to the boiler combustion chamber through which in use, the diverted water returning from the radiators is forced to pass to be cooled thereby.
 27. A building and heating system as claimed in claim 16 wherein the secondary condensing heat exchange means comprises two separate heat exchangers, a first in which the hot flue gases are cooled by water flowing therethrough to a temperature which is above the dew point temperature and a second through which the previously cooled flue gases pass and in which they are cooled to below the dew point temperature before escaping to atmosphere, and in which the second heat exchanger includes primary and secondary water circuits, water flowing around the secondary circuit in use being cooled by water flowing through the primary circuit, wherein the secondary circuit water is in contact with the flue gases to cool them below the dew point, and wherein water from the secondary circuit in use is caused to pass through the heat exchanger in the air inlet, to warm the incoming air and cool the secondary circuit water, before it returns to the secondary circuit and further comprising a secondary circuit pump which operates to pump secondary circuit water through the second heat exchanger and through the heat exchanger in the air inlet to the building and further comprising diverting means by which in use some of the secondary circuit water is caused to circulate through the heat exchanger in the air inlet to the building before it returns to the secondary circuit, and the remainder of the water is simply circulated around the secondary circuit by the secondary circuit pump.
 28. A building and heating system as claimed in claim 16 wherein the secondary condensing heat exchange means comprises two separate heat exchangers, a first in which the hot flue gases are cooled by water flowing therethrough to a temperature which is above the dew point temperature and a second through which the previously cooled flue gases pass and in which they are cooled to below the dew point temperature before escaping to atmosphere, and in which the second heat exchanger includes primary and secondary water circuits, water flowing around the secondary circuit in use being cooled by water flowing through the primary circuit, wherein the secondary circuit water is in contact with the flue gases to cool them below the dew point, and wherein water from the secondary circuit in use is caused to pass through the heat exchanger in the air inlet, to warm the incoming air and cool the secondary circuit water, before it returns to the secondary circuit and further comprising diverting means whereby in use some of the water returning from the radiators in the building to the boiler, is diverted through at least one further heat exchange means to be cooled therein before being supplied to an inlet to the primary circuit and further comprising an overall control system which includes temperature sensors and/or flow sensors and electrically controlled valves, whereby in use the volume and/or rate of flow of any diverted water associated with the secondary or the primary circuits is control.
 29. A building and heating system as claimed in claim 16 wherein the secondary condensing heat exchange means comprises two separate heat exchangers, a first in which the hot flue gases are cooled by water flowing therethrough to a temperature which is above the dew point temperature and a second through which the previously cooled flue gases pass and in which they are cooled to below the dew point temperature before escaping to atmosphere, and in which the second heat exchanger includes primary and secondary water circuits, water flowing around the secondary circuit in use being cooled by water flowing through the primary circuit, wherein the secondary circuit water is in contact with the flue gases to cool them below the dew point, and wherein water from the secondary circuit in use is caused to pass through the heat exchanger in the air inlet, to warm the incoming air and cool the secondary circuit water, before it returns to the secondary circuit and further comprising diverting means whereby in use some of the water returning from the radiators in the building to the boiler, is diverted through at least one further heat exchange means to be cooled therein before being supplied to an inlet to the primary circuit and wherein a first pump circulates heated water from the boiler through the radiators and back to the boiler and a second pump is provided which in use pumps the diverted water through the or each further heat exchanger and through the primary circuit, to allow greater control over the flow rate of the diverted returning water. 